JP2006054108A - Cathode for arc discharge and ion source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cathode for arc discharge generating arc discharge while emitting thermion from a cathode by making heating current flow through the cathode and generating plasma for obtaining targeted ion under the arc discharge, capable of generating plasma stably providing targeted ion for long period of time comparing with conventional this kind of cathode for arc discharge, and to provide an ion source adopting the same. <P>SOLUTION: The cathode for arc discharge 10 contains a center conductor part 1, a cylindrical conductor part 2 fitted to it from outside, and a connection conductor part 3 connecting tip parts of the both and has heating current flow in reverse directions at the center conductor part 1 and the cylindrical conductor part 2, and the ion source A adopting such a cathode 10. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an arc discharge cathode and an ion source equipped with the cathode.

  Arc discharge has been used in various fields. Today it is widely used in the field of ion engineering technology. For example, it is used to generate plasma for obtaining target ions in an ion source used in ion beam mixing technology that combines ion implantation, ion doping, ion implantation and thin film formation technology by PVD method, CVD method, etc. Has been.

  In an ion source that uses arc discharge to generate plasma, electrons necessary to maintain the discharge are stably emitted from the cathode, and there are deposits that cause discharge defects at the cathode and thus reduce plasma generation efficiency. In order to suppress the generation of a cathode or the like under plasma irradiation, the cathode is heated and held at a high temperature.

  As an example of the plasma generation rate reduction due to deposit generation, when the ion species extracted from the ion source is used for semiconductor impurity doping, ion implantation, etc., the gas supplied to the ion source or the vapor from the evaporation source (Hereinafter, these may be collectively referred to as “gas”.) In some cases, deposits may be generated on the surface of the cathode or the like. Such deposits may straddle between the cathode and other members and break the insulation between the two, thus preventing plasma generation.

  As an ion source for generating plasma using such arc discharge, for example, those described in Japanese Patent Application Laid-Open No. 2003-249177 can be cited. In the ion source of the type disclosed in the publication, a wire of a refractory metal such as tungsten having a uniform thickness (hereinafter sometimes referred to as “filament”) installed in a container surrounding a plasma generation region. In this case, a current is applied directly to the electrode, and resistance heating is performed to heat the thermoelectron emission to a temperature at which it becomes active. Further, a voltage is applied to the filament so as to have a negative potential with respect to the container. Generate and maintain

  At the beginning of the arc discharge, the thermoelectrons emitted from the filament are generally determined by a magnetic field generated around the filament based on the filament heating current and an electric field between the filament and the container, which is generally a Larmor having a very small radius of curvature. While swirling, it collides with a neutral gas introduced into the container in order to obtain target ions, causing an ionization reaction and generating plasma. After the plasma is generated, the thermoelectrons emitted from the heated filament are bent in a trajectory by a magnetic field for heating the filament, and in a region called a thin sheath generated between the plasma and the filament. Is electrostatically accelerated and released into the plasma. Electrons and ions in the plasma disappear due to diffusion, but new thermoelectrons are emitted from the filament, and the new thermoelectrons collide with neutral gas to generate new electrons / ions. The electrons and ions that disappear due to diffusion are compensated. Thus, the plasma is maintained.

  Since the plasma generated in this way is generated isotropically around the filament, the shape of the plasma is usually determined, ion diffusion is improved by suppressing plasma diffusion, and ion generation efficiency of the ion source is increased. Therefore, a linear magnetic field perpendicular to the ion extraction direction is applied from the outside. In general, the magnetic field is not limited to the shape and application direction.

  Other examples of the ion source include those described in JP-A-10-199430. In this ion source, the thermionic emission portion is disposed in a region exposed to plasma, while the heated portion for heating the portion is disposed in a region not exposed to plasma.

JP 2003-249177 A Japanese Patent Laid-Open No. 10-199430

  However, when a current is directly applied to the filament to heat the filament and emit thermoelectrons, the temperature distribution on the filament surface is not uniform.

  Since an arc current corresponding to the thermoelectron current emitted from the filament is superimposed on the filament, the current flowing through the filament is large at the cathode, so that a magnetic field is formed around the filament based on the heating current passed through the filament, The thermoelectrons emitted from the filament due to this magnetic field exist in the very vicinity of the filament because of the Larmor rotation with a very small radius of curvature. For this reason, the ionization probability due to collision with the gas increases on the negative electrode side of the filament where the potential difference increases, and plasma tends to be intensively generated on the negative electrode side. As a result, the effect of integrating the discharge current flowing from the plasma into the filament with thermionic emission into the filament heating current is integrated into the cathode toward the anode side of the filament. The side is heated and becomes concentrated in the vicinity of the negative electrode side end of the filament region exposed to the plasma.

  Thus, the negative electrode side end exposed to the filament plasma is easily heated to a higher temperature than the other parts, and the thermoelectrons are intensively emitted from that part, so that the region corresponding to the filament negative electrode side end is discharged. A high density plasma is intensively formed.

  Also, in the normal case, the filament is heated to near the melting point in order to increase the efficiency of thermionic emission, and is worn out by releasing the material from the filament surface by sputtering. This wear rate becomes larger on the filament negative electrode side due to the temperature non-uniformity accompanying the concentration of the heating rate, and eventually this portion is broken.

  More serious problems occur when there is a linear magnetic field as described above. As described above, when the filament negative electrode side is locally heated, more thermoelectrons are emitted from that portion than other portions. Thermionic emission localized by this intensive heating promotes ionization only in the region along the magnetic field lines, and as a result, a thin and high electron density region is generated in the plasma. This local high-density plasma generates positive feedback that further heats the original filament locally at the contact portion.

Eventually, the temperature of the filament high temperature part is determined by the value determined by the cooling rate by infrared radiation, and it becomes a steady state, but almost all the thermoelectrons are concentrated and emitted from a part on the filament, and the sublimation rate is also large. Thus, the filament life is significantly shortened.
In addition, this local heating occurs in a portion of the filament that is selectively subjected to plasma irradiation, and this heating portion combines the magnetic field applied from the outside with the magnetic field that the filament itself creates by the heating current, and Since it is determined by heat conduction through the filament, it also affects the spatial distribution and stability of the plasma.

  The above phenomenon is particularly serious when it is necessary to generate plasma as uniform as possible in the entire predetermined space corresponding to the cathode, making it difficult to achieve both the life and efficiency of the plasma equipment.

  In the ion source disclosed in Japanese Patent Laid-Open No. 10-199430, a thermionic emission source is separated into a heated portion and a plasma exposed portion so that a heated portion that tends to cause temperature non-uniformity at a high temperature is not directly exposed to plasma. Therefore, the maintenance interval can be extended. However, since the electron emission part for emitting thermoelectrons is not directly heated, the heating efficiency is poor. In addition, since the heat input diffuses in the electron emission part exposed to the plasma, the area of the thermal electron emission to the plasma in the electron emission part becomes too large, and as a result, it is difficult to generate plasma concentrated in a predetermined space region. Thus, the efficiency of plasma generation is reduced such that a preferable ion species density in ion extraction is concentrated in a predetermined space region.

  Therefore, the present invention applies a discharge voltage between the anode and the cathode for arc discharge, and causes a heating current to flow to the cathode to generate arc discharge while emitting thermoelectrons from the cathode. An arc discharge cathode for generating plasma for obtaining target ions, which can generate plasma that can stably provide desired ions over a long period of time as compared with a conventional arc discharge cathode of this type. It is an object of the present invention to provide an arc discharge cathode that can be used.

  The present invention also applies a discharge voltage between an anode and a cathode for arc discharge, and causes a heating current to flow through the cathode to generate arc discharge while emitting thermoelectrons from the cathode. An ion source that generates a plasma for obtaining target ions and extracts the ions from the plasma, and is capable of emitting ions that are stably required over a long period of time compared to a conventional ion source of this type It is an issue to provide.

In order to solve the above problems, the present invention provides the following arc discharge cathode and ion source.
(1) Arc discharge cathode A discharge voltage is applied between the anode and the cathode for arc discharge, and a heating current is applied to the cathode to cause the generation of arc discharge while emitting thermoelectrons from the cathode. And a central conductor portion comprising a cylindrical conductor portion and one or a plurality of conductors disposed inside the cylindrical conductor portion, the cathode for arc discharge generating plasma for obtaining target ions. And a connecting conductor portion that electrically connects the cylindrical conductor portion and the tip end portion of the central conductor portion, and for arc discharge in which a cathode heating current flows in the opposite direction between the cylindrical conductor portion and the central conductor portion cathode.

(2) Ion source A discharge voltage is applied between the anode and the cathode for arc discharge, and an arc discharge is generated while flowing a heating current to the cathode to emit thermoelectrons from the cathode. Is an ion source that generates plasma for obtaining target ions and extracts the ions from the plasma, and the arc discharge cathode is a cylindrical conductor portion and one disposed inside the cylindrical conductor portion. Or a central conductor portion composed of a plurality of conductors and a connecting conductor portion that electrically connects the cylindrical conductor portion and the tip end portion of the central conductor portion, and the cylindrical conductor portion and the central conductor portion are heated by cathode. An ion source that is a cathode in which a current flows in the opposite direction (in short, an ion source that employs the arc discharge cathode described in (1) above).

  In the arc discharge cathode according to the present invention and the arc discharge cathode in the ion source according to the present invention, the cathode heating current flows in the opposite direction between the cylindrical conductor part constituting the cathode and the central conductor part disposed therein. Therefore, a magnetic field formed based on the current flowing in the central conductor portion and a magnetic field formed based on the current flowing in the opposite direction to the cylindrical conductor portion are generated, and these magnetic fields cancel each other, and the entire cathode As a result, the formation of the magnetic field around the cathode based on the cathode heating current is sufficiently suppressed.

As a result of the suppression of the magnetic field formation around the cathode in this way, thermoelectrons are easily emitted from the entire outer cylindrical conductor, and it corresponds to the cylindrical conductor when no magnetic field is applied to the plasma generation region from the outside. The plasma is formed over the entire area, so that an inflow of current from the plasma to the cathode is realized for the entire cathode, which suppresses the local overheating of the cathode, and thus the life of the cathode is lengthened. It is possible to generate plasma for obtaining ions that are stably obtained over a wide range.
Further, when a linear magnetic field is applied from the outside along the length direction of the cathode, the emitted thermoelectrons are bound to the linear magnetic field and move in the length direction of the cathode. For this reason, plasma is generated in a concentrated manner in the connecting conductor portion and the portion including the central conductor portion and the tip portion of the cylindrical conductor portion (hereinafter also referred to as “cathode tip portion”). For this reason, inflow of current from the plasma to the cathode is concentrated on the tip of the cathode, and local heating of the tip of the cathode occurs, but by making the thickness of the connecting conductor portion sufficiently thicker than the thickness of the cylindrical conductor portion. The cathode life can be extended to a desired time.

  Further, according to the present invention, plasma having conditions necessary for obtaining target ions is generated by adjusting the heating rate of the central conductor part and the cylindrical conductor part of the arc discharge cathode. Therefore, it is possible to adjust the heating efficiency of the cathode, the discharge efficiency of arc discharge, etc. (for example, adjustment for increasing the efficiency) corresponding to various operating conditions.

More specifically, according to the desired plasma state for obtaining the target ions, for the heating surface temperature distribution adjustment (typically uniform temperature distribution adjustment) of the cylindrical conductor when the cathode is used. Furthermore, at least one of the electrical resistance of each part of the cylindrical conductor part, the electrical resistance of each part of the central conductor part, and the electrical resistance of the connection conductor part can be adjusted.
For the purpose of adjusting the electrical resistance, for example, at least one of the thickness of each part of the cylindrical conductor part, the thickness of each part of the central conductor part, and the volume of the connection conductor part can be adjusted. In addition to or in place of such adjustment of the thickness of each part of the cylindrical conductor part, the shape of the cylindrical conductor part (cross-sectional shape etc.), the shape of the central conductor part (cross-sectional shape etc.) and It is also possible to adjust at least one of the shapes (cross-sectional shapes, etc.) of the connection conductor portion.

  More specifically, according to the arc discharge cathode according to the present invention and the arc discharge cathode in the ion source according to the present invention, the cathode heating current is transferred from the root portion of the cylindrical conductor portion to the tip portion, and further to the connection conductor. The cathode heating rate from the plasma irradiation is concentrated on the cathode tip using the cathode tip exposed to the plasma at the most negative potential. Can do.

  In this case, even if the cathode heating current is made to flow uniformly to each conductor, an electric resistance is given to the cathode tip so that the surface temperatures of the cathode tip and the base portion of the cylindrical conductor are almost the same. The cathode tip portion may be formed. For example, it can be realized by forming the cathode tip portion thicker than other portions of the cylindrical conductor portion and the central conductor portion. By so doing, the temperature distribution on the cathode surface (tubular conductor surface) can be made more uniform, and the cathode life can be extended by itself.

  If the rated discharge current is determined, a part of the cathode heating power can be added to the discharge current by appropriately selecting the thickness distribution of the outer cylindrical conductor and the thickness distribution of the center conductor. It is also possible to improve the heating efficiency of the cathode without sacrificing the life of the cathode.

  When it is desired to use the arc discharge cathode according to the present invention in a wide discharge current range, in other words, when it is desired to select the plasma density from a wide range, for example, the cathode heating rate by the cathode heating current is from plasma to cathode. In other words, the rate of thermionic emission from the cathode can be easily controlled by adjusting the cathode heating power without being significantly affected by the discharge current. By doing so, it is possible to form plasma having a desired density and size while maintaining uniform surface temperature distribution of each part of the cylindrical conductor part even if the thickness of each part of the outer cylindrical conductor part is substantially constant. Thus, the said cylindrical conductor part may be formed so that the thickness of each part is constant.

  In addition, the central conductor portion, the cylindrical conductor portion, and the connection conductor portion can be formed of the same material, or the cylindrical conductor portion is formed of a higher resistance material than the central conductor portion in order to improve thermionic emission efficiency. You can also

In any case, the center conductor portion, the cylindrical conductor portion, and the connection conductor portion are preferably formed from a high melting point material, but examples of the high melting point material include the following.
That is, a single metal material selected from molybdenum, tantalum, tungsten, rhenium, iridium, or a metal material made of an alloy containing at least two of these metals, or using the metal material as a base material, the base material as an alkali metal, It is a material coated with one of alkaline earth metal, alkali metal oxide, alkaline earth metal oxide, alkali metal carbide, and alkaline earth metal carbide.

  As a representative example of the shapes of the center conductor portion, the cylindrical conductor portion, and the connection conductor portion, the center conductor portion may be a rod-like conductor portion having a circular cross section, and the cylindrical conductor portion may be a cylindrical conductor portion. it can. The rod-shaped central conductor portion may be composed of a single rod-shaped conductor or may be formed by bundling a plurality of rod-shaped conductors. The central conductor portion may be a coiled conductor instead of a rod-shaped conductor.

In addition, examples of the connection conductor portion include a ring-shaped conductor portion that is externally fitted to the central conductor portion and is fitted to the cylindrical conductor portion. Such a ring-shaped connecting conductor portion can be formed by machining the material of the conductor portion, and a sheet-like conductor material is wound around the central conductor portion, and a cylindrical conductor portion is externally fitted thereto, or further In this state, it can be formed integrally with the central conductor portion and the cylindrical conductor portion by electron beam welding or laser welding.
Note that the shapes of the central conductor portion, the cylindrical conductor portion, and the connection conductor portion are not limited to those described above, and various shapes can be adopted within a range that does not hinder. For example, the central conductor portion and the cylindrical conductor portion do not need to have a circular cross-sectional outline, and may be polygonal in a range that does not hinder, for example. Further, the connection conductor portion is not limited to a circular ring shape, and may be of an appropriate shape according to the cross-sectional shape of the center conductor portion or the cylindrical conductor portion.

  As described above, according to the present invention, a discharge voltage is applied between the anode and the cathode for arc discharge, and a heating current is applied to the cathode to generate thermoelectrons from the cathode, thereby generating arc discharge. An arc discharge cathode for generating a plasma for obtaining desired ions under discharge, which can provide ions that are stably required over a long period of time compared to a conventional arc discharge cathode of this type. It is possible to provide an arc discharge cathode capable of generating plasma.

  According to the present invention, a discharge voltage is applied between the anode and the cathode for arc discharge, and a heating current is applied to the cathode to generate arc discharge while emitting thermoelectrons from the cathode. An ion source that generates plasma for obtaining target ions in the plasma and extracts the ions from the plasma, and can perform ion emission that can be stably obtained over a long period of time compared to this type of conventional ion source Can be provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows one example (ion source A) of an ion source employing one example of an arc discharge cathode according to the present invention. FIG. 2 shows the cross-sectional structure of the cathode.
In the ion source A, an arc discharge cathode 10 is installed in a container C surrounding a plasma generation region, an ion extraction electrode E is installed opposite to an opening of the container C, and the ion source A is constituted by a permanent magnet or an electromagnet outside the container C. The container C of the ion source A in the illustrated example is further disposed in the vacuum container CH, and the inside of the vacuum container CH is decompressed by an exhaust device (not shown), The inside of the container C is also decompressed. Moreover, the magnet M is installed with respect to the outer periphery of the container C through the vacuum container CH. The vacuum container CH is a container that accommodates, for example, a substrate to be ion-implanted in an ion implantation apparatus facing the ion source.

The container C of the ion source A is provided with a gas or vapor inlet G, from which gas or vapor for obtaining target ions can be introduced into the container C. .
The magnet M forms a linear magnetic field in the container C perpendicular to the ion extraction direction X by the ion extraction electrode system E and in the direction Y along the arc discharge cathode 10 in order to confine the plasma formed in the container C.

Although the details of the cathode 10 will be described later, the cathode 10 is installed in the container C so as to face the ion extraction electrode system E in a posture parallel to the electrode system E, and is connected to a power source PW1 for heating the cathode. The container C also serves as an anode for the arc discharge cathode 10, and a power source PW <b> 2 for arc discharge is connected to the container C and the cathode 10.
A power supply PW3 for applying a potential for ion extraction is connected between the ion extraction electrode system E and the container C.

  As shown in FIG. 2, the arc discharge cathode 10 includes a rod-shaped central conductor portion 1 having a circular cross section, a cylindrical conductor portion 2 that is externally fitted with a space therebetween, and a tip portion and a cylinder of the central conductor portion 1. A ring-shaped connecting conductor portion 3 for connecting the tip end portion of the shape conductor portion 2 is included.

The cylindrical conductor portion 2 passes through the electrical insulator IS provided on the wall of the container C and is drawn out of the container, and the cylindrical holding conductor portion 21 fitted to the insulator IS is the cylindrical conductor portion. The base part of 2 is externally connected. The cylindrical conductor portion 2 is connected to the positive electrode of the cathode heating power source PW1 through the holding conductor portion 21.
At least a portion of the electrical insulator IS located in the container C and exposed to the plasma is formed of alumina ceramic, boron nitride, or the like. This protects the holding conductor portion 21 that is not necessarily formed of a refractory metal from direct plasma irradiation.

  The base portion of the center conductor portion 1 is held by the holding conductor portion 11, and the holding conductor portion 11 is connected to the negative pole of the power source PW1. The holding conductor portion 11 is supported while being insulated from the container C wall or the vacuum container CH through a member (not shown).

The cathode 10 will be described in further detail. The central conductor 1, the cylindrical conductor 2, and the connecting conductor 3 are made of tantalum, which is the same refractory metal material here.
The cylindrical conductor portion 2 has a uniform thickness in the range of about 0.1 mm to 1 mm throughout, and is formed in the range of an outer diameter of about 3 mm to 7 mm and a length of about 5 mm to 30 mm.
The central conductor portion 1 is formed in a form in which rods or fine wires having an outer diameter in the range of about 0.5 mm to 3 mm are bundled, and the length is longer than that of the cylindrical conductor portion 2.

The connecting conductor 3 in the cathode 10 may be merely fitted to the central conductor 1 and the cylindrical conductor 2 in a state of being sufficiently closely contacted with the central conductor 1 and the cylindrical conductor 2. It is desirable to be integrated with the cylindrical conductor 2. In this example, they are integrated by electron beam welding.
Here, the connection conductor 3 is formed to be sufficiently thicker than the outer diameter of the central conductor 1 and the wall thickness of the cylindrical conductor 2, so that a long life can be obtained by sputtering by plasma concentrated on the cathode tip e. It is.

  When general tungsten is used as the cathode material in the present invention, when the connecting conductor portion 3 is formed by machining, the tungsten material is heated to a temperature equal to or higher than 300 ° C. which is the malleable ductile transition temperature. However, in order to avoid this, a sheet-like tungsten material is wound around the tip of the central conductor portion 1, and the tip of the cylindrical conductor portion 2 is fitted to the tip to tentatively assemble them. You may integrate by welding, laser welding, etc.

  When tantalum, rhenium or the like is adopted as the cathode material, these materials can be machined. Therefore, the connecting conductor portion 3 having a precise shape is formed using a lathe or the like, particularly, electron beam welding or laser. Even if welding is not employed, the cathode 10 can be assembled by fitting it to the central conductor portion 1 and the cylindrical conductor portion 2 by a simple fitting process.

  The connection between the central conductor portion 1 and the holding conductor portion 11 and the connection between the cylindrical conductor portion 2 and the holding conductor portion 21 are required to sufficiently consider the influence of thermal expansion. In normal operation, the center conductor portion 1 has a higher temperature than the outer cylindrical conductor portion 2, and this is taken into consideration, and the connecting portion may be designed in consideration of the expansion difference between the conductors.

  With respect to the center conductor portion 1, the center conductor portion 1 and its holding conductor portion 11 are slidably fitted to the holding conductor portion 11 when the interconnection between the center conductor portion 1 and the holding conductor portion 11 is closely fitted. We conducted a power-on test for two types of cases, and both worked without problems. The center conductor portion 1 and the connecting conductor portion 3 were also tested with both the one connected by electron beam welding and the one with a fitting structure, but all functioned without hindrance.

  In order to remove residual stress that may be generated during welding in the assembly of the cathode, the temperature of the cathode 10 is raised to 1 ° C. per second when the cathode 10 is first heated to the electron emission temperature. The temperature was slowly raised with a temperature gradient of less than about. By doing so, even if the temperature was raised in a relatively short time thereafter, the adverse effect due to the residual stress on the cathode was sufficiently suppressed.

  According to the ion source A described above, the inside of the vacuum vessel CH is evacuated from the inside of the vessel C of the ion source A by evacuating the inside of the vacuum vessel CH by an unillustrated exhaust device, and the plasma generation gas pressure is maintained at about 10 Pa or less, for example A predetermined amount of gas or vapor for obtaining ions is introduced into a container C through a gas or vapor inlet port G from a gas supply device or vapor generator (not shown), and the cathode 10 is heated by a power source PW1. A plasma is generated in the container C by applying a discharge voltage from the power source PW2 between the cathode 10 and the container C while discharging current from the cylindrical conductor portion 2 by flowing current, and an ion extraction electrode system from the plasma. An ion beam can be extracted by E.

  At this time, in the arc discharge cathode 10, the cathode heating current is caused to flow in the opposite direction between the central conductor portion 1 constituting the arc discharge cathode and the cylindrical conductor portion 2 fitted on the central conductor portion 1. And a magnetic field based on a current flowing in the opposite direction to the cylindrical conductor portion 2 is generated, and these magnetic fields cancel each other, so that the cathode 10 as a whole is around the cathode based on the cathode heating current. The formation of the magnetic field is sufficiently suppressed.

  As a result of suppressing the formation of the magnetic field around the cathode in this way, the thermal electrons are easily emitted from the entire outer cylindrical conductor portion 2, and along the linear magnetic field formed by the magnet M installed outside the container C. The plasma is concentrated and generated in the vicinity of the connection conductor portion 3 having the lowest potential on the surface of the cathode 10. For this reason, the inflow of current from the plasma to the cathode 10 is concentrated on the connecting conductor portion 3 having a thickness larger than that of the cylindrical conductor portion 2, and the lifetime of the cathode 10 is exposed to the plasma. Longer than. Moreover, since the thickness of the connection conductor part 3 can be made thicker than that of a rod-like cathode having a circular cross section that is generally used, the cathode life is longer than that of a general cathode. Therefore, by using the cathode 10, it is possible to generate plasma for obtaining ions stably obtained over a long period of time, and perform the ion emission.

  Further, in this example, as shown in the figure, the cathode heating current from the power source PW1 is transferred from the root part of the cylindrical conductor part 2 to the tip part, and further from the tip part of the center conductor part 1 to the root part via the connection conductor part 3. As a result, the cathode tip e including the connection conductor portion 3 has the most negative potential, and the cathode heating due to plasma irradiation concentrates on the cathode tip e. Here, as described above, the connection conductor portion 3 is formed to be sufficiently thicker than the outer diameter of the central conductor portion 1 and the thickness of the cylindrical conductor portion 2 to reduce the electrical resistance of the cathode tip portion e, and the cathode tip portion is also the same as the root portion of the cylindrical conductor portion 2. Since the surface temperature is heated to a certain level, the cathode surface temperature can be made more uniform in combination with the suppression of the magnetic field formation.

  Here, the ratio of cathode heating by the cathode heating current by the power source PW1 is set to be larger than the ratio of cathode heating by the discharge current flowing from the plasma into the cathode 10, and it is not greatly affected by the discharge current. The ratio of thermionic emission from the cathode can be easily controlled by adjusting the cathode heating power, thereby controlling the discharge current in a wide range and forming plasma with a desired density.

Next, an experiment for examining the discharge characteristics of the cathode 10 will be described.
In the experiment, the outer diameter of the central conductor portion 1 is 1.0 mm, the cylindrical conductor portion 2 has a uniform thickness of 0.1 mm, the outer diameter is 3.2 mm, the length of the central conductor portion 1 is 32 mm, the cylindrical conductor portion. A cathode having a length of 25 mm and a thickness of the connecting conductor portion 3 of 1.5 mm was employed. Residual stress was sufficiently removed in advance.

  While the magnetic field by the magnet M in the container C is about 80 Gauss and the gas pressure is maintained at 0.15 Pa, argon gas is introduced into the container C and various heating currents are supplied to the cathode by the power source PW1, while the power source When the electron current Id [A] emitted from the cathode with the application of the discharge voltage Vd [V] by PW2 was measured, the result shown in FIG. 3 was obtained.

  As shown in FIG. 3, the discharge started sharply when the discharge voltage was around 13.5 V, which is the ionization voltage of argon. This is significantly different from the discharge current characteristics when using a conventional wire filament in which a large magnetic field is formed on the cathode surface by the cathode heating current. Typical characteristics seen when there is no local magnetic field on the cathode surface It is. Therefore, it is shown that stable plasma can be maintained even with a very low discharge voltage and discharge current. When the shape of the generated plasma was observed, it was confirmed that the plasma was formed in a size substantially corresponding to the size of the cathode.

  In addition, when the infrared radiation intensity distribution of the cathode was examined with an infrared camera, it was found that almost uniform infrared radiation was generated from each part of the cathode, and a substantially uniform temperature distribution corresponding to that was realized.

  INDUSTRIAL APPLICABILITY The present invention can be used to stably generate desired ions for a long time in various ion devices.

It is a figure which shows the outline of a structure of one example of the ion source which employ | adopted one example of the cathode for arc discharges concerning this invention. . It is a figure which shows the cross-section of the arc discharge cathode shown in FIG. It is a figure which shows an experimental result.

Explanation of symbols

A ion source 10 arc discharge cathode 1 central conductor portion 11 holding conductor portion 2 cylindrical conductor portion 21 holding conductor portion 3 connecting conductor portion e cathode tip C ion source vessel E ion extraction electrode system M magnet G introduction of gas or vapor Mouth IS Electrical insulator PW1 Cathode heating power supply PW2 Discharge power supply PW3 Ion extraction power supply CH Vacuum container

Claims (13)

  A discharge voltage is applied between the anode and the cathode for arc discharge, and a heating current is applied to the cathode to generate arc discharge while emitting thermal electrons from the cathode. The arc discharge cathode for generating plasma for obtaining a center conductor portion, a cylindrical conductor portion externally fitted in a state of being electrically insulated from the central conductor portion, and the central conductor portion and the cylindrical shape An arc discharge cathode comprising: a connecting conductor portion for electrically connecting a tip portion of the conductor portion, wherein the heating current is caused to flow in the opposite direction between the central conductor portion and the cylindrical conductor portion.   At least one of the electrical resistance of each part of the cylindrical conductor part, the electrical resistance of each part of the central conductor part, and the electrical resistance of the connection conductor part is adjusted for adjusting the heating surface temperature distribution of the cylindrical conductor part when the cathode is used. The arc discharge cathode according to claim 1.   At least one of the thickness of each part of the cylindrical conductor part, the thickness of each part of the central conductor part, and the volume of the connecting conductor part is adjusted to adjust the heating surface temperature distribution of the cylindrical conductor part when the cathode is used. The arc discharge cathode according to claim 1.   The arc discharge cathode according to claim 1, wherein the cylindrical conductor portion has a constant thickness at each portion.   The arc discharge cathode according to any one of claims 1 to 4, wherein the central conductor portion, the cylindrical conductor portion, and the connection conductor portion are formed of the same material.   The cathode for arc discharge according to any one of claims 1 to 4, wherein the cylindrical conductor portion is formed of a higher resistance material than the central conductor portion.   The central conductor portion, the cylindrical conductor portion, and the connection conductor portion are each formed from a high melting point material, and the high melting point material includes at least two of molybdenum, tantalum, tungsten, rhenium, iridium, and these metals. A metal material selected from among alloys, or using the selected metal material as a base material, and using the base material as an alkali metal, alkaline earth metal, alkali metal oxide, alkaline earth metal oxide, alkali metal The power source for arc discharge according to any one of claims 1 to 6, wherein the power source is a material coated with one of carbide and alkaline earth metal carbide.   The arc discharge cathode according to any one of claims 1 to 7, wherein the central conductor portion is a rod-like conductor portion having a circular cross section, and the cylindrical conductor portion is a cylindrical conductor portion.   The cathode for arc discharge according to any one of claims 1 to 7, wherein the central conductor portion is a bundle of rod-shaped conductors, and the cylindrical conductor portion is a cylindrical conductor portion.   The arc discharge cathode according to any one of claims 1 to 9, wherein the connection conductor portion is a ring-shaped conductor portion that is externally fitted to the central conductor portion and is fitted to the cylindrical conductor portion.   A discharge voltage is applied between the anode and the cathode for arc discharge, and a heating current is applied to the cathode to generate arc discharge while emitting thermal electrons from the cathode. An ion discharge cathode according to any one of claims 1 to 10 is employed as the arc discharge cathode. Ion source.   The ion source according to claim 11, wherein the cathode heating current is caused to flow from a root portion of the cylindrical conductor portion to a tip portion, and further from the tip portion of the center conductor portion to the root portion via the connection conductor portion. The ion source according to claim 11, wherein a part of the cylindrical conductor heating power is carried by a discharge current flowing from the plasma to the cathode.